If ReadCompaction is switched off, then it is better to not even submit background compaction jobs.

Summary:
If ReadCompaction is switched off, then it is better to not even
submit background compaction jobs. I see about 3% increase in
read-throughput on a pure memory database.

Test Plan: run db_bench

Reviewers: heyongqiang

Reviewed By: heyongqiang

Differential Revision: https://reviews.facebook.net/D5673

db/db_impl.cc

@@ -1196,7 +1196,8 @@ Status DBImpl::Get(const ReadOptions& options,
     mutex_.Lock();
   }
 
-  if (have_stat_update && current->UpdateStats(stats)) {
+  if (!options_.disable_seek_compaction &&
+      have_stat_update && current->UpdateStats(stats)) {
     MaybeScheduleCompaction();
   }
   mem->Unref();

db/version_set.cc

@@ -1392,7 +1392,7 @@ Compaction* VersionSet::PickCompaction() {
       // Wrap-around to the beginning of the key space
       c->inputs_[0].push_back(current_->files_[level][0]);
     }
-  } else if (seek_compaction && !options_->disable_seek_compaction) {
+  } else if (seek_compaction) {
     level = current_->file_to_compact_level_;
     c = new Compaction(level, MaxFileSizeForLevel(level),
 		MaxGrandParentOverlapBytes(level), NumberLevels());